Process for preparing pure anastrozole

ABSTRACT

The present invention discloses two new related substances (6) and (7) of Anastrozole synthesis from Q.A. Salt (5) as in Scheme-1 and purification procedures to get Anastrozole (1) free from (6) and (7).

FIELD OF INVENTION

Aromatase is an enzyme, which effects aromatisation of ring A in themetabolic formation of various steroid hormones. Various cancers, forexample, breast cancer are dependent upon circulating steroid hormones,which have an aromatic ring A. Such cancers can be treated by removingthe source of ring A aromatised steroid hormones, for example, by thecombination of oophorectomy and adrenalectomy. An alternative way ofobtaining the same effect is by administering a chemical compound, whichinhibits the aromatisation of the steroid ring A.

Anastrozole is a non-steroidal antineoplastic, claimed to inhibit thearomatase (oestrogen synthase) activity. It is useful in the treatmentof advanced breast cancer in postmenopausal women.

BACKGROUND OF INVENTION

Synthesis of Anastrozole is reported in U.S. Pat. No. 4,935,437 andEuropean Patent Application EP 0,296,749. The synthetic route mentionedin the said patents suffers a major disadvantage of the formation ofAnastrozole regioisomer (2).

To overcome the formation of regioisomer (2), another synthetic route isreported in U.S. Pat. No. 4,935,437; in which compound (3) is reactedwith 4-amino-1,2,4-triazole (4) to form quaternary ammonium salt (5),which further undergoes diazotisation reaction to give Anastrozole (1)free from regioisomeric impurity (2) (Scheme-1).

It has been observed that the cyano groups undergo hydrolysis in variousconditions to form hydrolysed related compounds.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide an improved processfor the preparation of pure Anastrozole (1) free from impurities arisingdue to hydrolysis of cyano groups during the course of the preparationof Anastrozole (1).

DESCRIPTION OF INVENTION

Intermediate (3) undergoes condensation with 4-amino-1,2,4-triazole (4)in a suitable solvent to give4-amino-1-[3,5-bis-(1-cyano-1-methylethy)phenzyl]-1H-[1,2,4]triazoliumbromide (Q.A.-salt) (5) in good yield.

It has been further observed that during the preparation of Anastrozole,hydrolysis of cyano groups also takes place leading to the formation oftwo major related substances. The hydrolysis products formed due tohydrolysis are characterized as2-[3-(cyanodimethyl-methyl)-5-[1,2,4]triazol-1-ylmethyl-phenyl]-isobutyramide(6) and2-[3-(1-carbamoyl-1-methylethyl)-5-[1,2,4]triazol-1-ylmethylphenyl]-isobutyramide(7). Both the substances are isolated and well characterized by usingNMR and mass analysis. The ¹H-NMR, ¹³C-NMR and mass analysis of theisolated products2-[3-(cyanodimethyl-methyl)-5-[1,2,4]triazol-1-ylmethyl-phenyl]-isobutyramide(6) and2-[3-(1-carbamoyl-1-methylethyl)-5-[1,2,4]triazol-1-ylmethylphenyl]-isobutyramide(7) are in accordance with the chemical structure. ¹H-NMR of compound(6) shows three singlets at δ 7.4, 7.3 and 7.24 for three protons inaromatic ring, and two protons at δ 6.95 for amide group. However fourmethyl groups appear at δ 1.65 and 1.41, each for six protons. The¹³C-NMR of compound (6) shows a quaternary peak at δ 177.4 for amidecarbonyl carbon, three tertiary aromatic carbons at δ 125.0, 122.7 and122.2; two aliphatic quaternary carbons at δ 52.1 and 46.1 and two peaksfor methyl carbons at δ 28.4 and 26.7. Further, the structure is alsoconfirmed by the mass analysis of compound (6).

2-[3-(Cyano-dimethyl-methyl)-5-[1,2,4]triazol-1-ylmethyl-phenyl]isobutyramide

The 1H-NMR of compound (7) shows peaks at δ 6.86 for amide protons andits 13C-NMR shows amide carbonyl carbon at δ 179.6. Further, thestructure is also confirmed by the mass analysis of compound (7).

2-[3-(1-Carbamoyl-1-methylethyl)-5-[1,2,4]triazol-1-ylmethylphenyl]-isobutyramide

The HPLC chromatogram of Anastrozole shows presence of relatedsubstances (6) and (7) in 0.02% to 1.0% in crude product which areremoved by the repeated crystallization using an alcoholic solvent witha mixture of hydrocarbon as anti-solvent.

The removal of the related substances2-[3-(cyanodimethyl-methyl)-5-[1,2,4]triazol-1-ylmethyl-phenyl]-isobutyramide(6) and2-[3-(1-carbamoyl-1-methylethyl)-5-[1,2,4]triazol-1-ylmethylphenyl]-isobutyramide(7) are accomplished by the crystallization method using various solventsystems to get Anastrozole in its purer form. Thus, the main embodimentof the present invention relates to the products2-[3-(cyanodimethyl-methyl)-5-[1,2,4]triazol-1-ylmethyl-phenyl]-isobutyramide(6) and2-[3-(1-carbamoyl-1-methylethyl)-5-[1,2,4]triazol-1-ylmethylphenyl]-isobutyramide(7) as related substances in Anastrozole. According to anotherembodiment, the present invention also relates to the process for thepreparation of Anastrozole with related substances2-[3-(cyanodimethyl-methyl)-5-[1,2,4]triazol-1-ylmethyl-phenyl]-isobutyramide(6) and2-[3-(1-carbamoyl-1-methylethyl)-5-[1,2,4]triazol-1-ylmethylphenyl]-isobutyramide(7) preferably, less than 1.0%, more preferably, 0.1% and mostpreferably, below quantitation limits.

NMR data of Anastrozole (1) and related substances (6) and (7) ¹H-NMR(DMSO-d₆) ¹³C-NMR (DMSO-d₆) Anastrozole 8.72 (s, 1H), 8.01 (s, 1H), 7.57(t, 151.9, 144.4, 142.7, 137.8, 124.4, 1H, J = 1.6 Hz), 7.46 (d, 2H, J =1.6 Hz), 124.2, 121.6, 51.8, 36.8 and 28.2. 5.51 (s, 2H) and 1.68 (s,12H). Related 8.88 (s, 1H), 7.99 (s, 1H), 7.40 (s, 177.4, 151.8, 147.5,144.3, 141.6, Substance (6) 1H), 7.30 (s, 1H), 7.24 (s, 1H), 136.7,125.0, 124.5, 122.7, 122.2, 6.95 (d, 2H, J = 9.9 Hz), 5.43 (s, 52.1,46.1, 36.7, 28.4 and 26.7. 2H), 1.65 (s, 6H) and 1.41 (s, 6H). Related8.61 (d, 1H, J = 6.9 Hz), 7.96 (d, 179.6, 153.5, 148.3, 146.0, 137.4,Substance (7) 1H, J = 6.2 Hz), 7.28 (s, 1H), 125.4, 124.9, 54.3, 47.9and 28.8. 7.14 (d, 2H, J = 1.4 Hz), 6.86 (d, 4H, J = 15.6 Hz), 5.38 (s,2H) and 1.39 (s, 12H).

Following the procedures as per Scheme-1 Anastrozole is obtained in itspurer form but still some extent of the related substances2-[3-(cyanodimethyl-methyl)-5-[1,2,4]triazol-1-ylmethyl-phenyl]-isobutyramide(6) and2-[3-(1-carbamoyl-1-methylethyl)-5-[1,2,4]triazol-1-ylmethylphenyl]-isobutyramide(7) still remain contaminating Anastrozole, which is further purifiedusing organic solvents preferably isopropanol, ethyl acetate or mixtureof solvents preferably cyclohexane/ethyl acetate,cyclohexane/isopropanol or a mixture of solvents with water. Thusanother embodiment of the present invention relates to the process forthe preparation of Anastrozole free from related substances (6) and (7)by crystallization of crude Anastrozole using alcohols preferablyselected from C1 to C10 alcohols and hydrocarbons, preferably selectedfrom aliphatic hydrocarbons preferably C1 to C10.

EXAMPLE-12,2′-[5-(1H-1,2,4-Triazol-1-ylmethyl)-1,3-phenylene]di(2-methylpropiononitrile)(1), Anastrozole

4-Amino-1-[3,5-bis-(1-cyano-1-methylethyl)benzyl]-1H-[1,2,4]triazoliumbromide (5) (70 g) was dissolved in conc. HCl (280 mL) in a 5 L R.B.flask and cooled to −5 ° C. A solution of sodium nitrite (15 g) in water(70 mL) was slowly added to the reaction mixture at 0-5° C. in 4 hrs andthe reaction mixture was stirred for one hour at 0-5° C. and further at10-20° C. for next 3 hours. The reaction mixture was quenched by theaddition of a solution of urea (4.5 g) in water (15 mL). Toluene (700mL) was added to the reaction mixture and the heterogeneous solution wasfurther cooled down to 0-5° C. The solution was basified by the additionof liquor ammonia (365 mL) slowly in 4 hours at 5-25° C. Organic layerwas separated and further washed with water (200 mL). Aqueous layer wasremoved and a solution of cone. HCl (140 mL) in water (140 mL) was addedto the organic layer slowly in 30 minutes at 25-30° C. and reaction masswas heated at 60-65° C. for 30 minutes. The lower aqueous layer (280-300mL), containing product was collected in a conical flask maintaining at50° C. The aqueous part was again washed with toluene (700 mL) at 60-65°C. for 30 minutes. The lower aqueous layer, containing product wascharged in a separating funnel and again washed with fresh toluene (700mL). The aqueous layer, containing product was transferred in a R.B.flask and ethyl acetate (350 mL) was added to it. The heterogeneoussolution was cooled to 0-5° C. basified by the slow addition of liquorammonia (280 mL) in 2-3 hours at 5-25° C. The solution was stirred forone hour at 25-35° C., and the upper organic layer (360-375 mL),containing product was separated and filtered through hyflow super cellbed. Solvent was distilled out below 50° C. under vacuum leavingapproximately 100 mL ethyl acetate in the flask. The content of theflask was cooled down to 25-35° C. and cyclohexane (500 mL) was added tothe solution slowly in 30 minutes. The precipitated solid product wasfiltered and washed with fresh cyclohexane (20 mL×2). The product wasdried at 45-50° C. to get crude Anastrozole (44 g) with more than 98%HPLC purity contaminated with related substance (6) as 0.36% and withrelated substance (7) as 0.05%.

EXAMPLE-2 Removal of Related Substances (6) and (7) from Anastrozole

Anastrozole (33 g) from example-2 was dissolved in isopropanol (100 mL)at 45-50° C. The solution was cooled down to 25-35° C. and cyclohexane(100 mL) was added drop wise in 30 minutes. The solution was stirred at25-35° C. for 2 hours; the precipitated solid product was filtered andwashed with fresh cyclohexane (30 mL×2) and dried at 50° C. to get 23 gof pure Anastrozole contaminated with related substance (6) as 0.09% andwith related substance (7) below detection limit.

EXAMPLE-3 Purification of Anastrozole

Pure Anastrozole (11 g) from Example-2 was further purified bydissolving in isopropanol (33 mL) at 45-50° C. The solution was cooleddown to 25-35° C. and cyclohexane (33 mL) was added drop wise in 30minutes. The solution was stirred at 25-35° C. for 2 hours; theprecipitated solid product was filtered and washed with freshcyclohexane (30 mL×2) and dried at 50° C. to get 8.9 g of pureAnastrozole containing with 0.03% of (6) as related substance andanother related substance (7) below detection limit. Related substance(6) can be further removed below detection limit by repeating the sameprocess.

1.-5. (canceled)
 6. A compound consisting essentially of Anastrozolewith related substances of structural formula (6) and (7) removedtherefrom,

the compound prepared by a process comprising the steps of: a.dissolving Anastrozole in an alcoholic solvent; b. adding hydrocarbon tothe alcoholic solution; and c. isolating pure Anastrozole.
 7. A compoundconsisting essentially of Anastrozole, the compound prepared by aprocess comprising the steps of a. dissolving Anastrozole in analcoholic solvent; b. adding hydrocarbon to the alcoholic solution; andc. isolating pure Anastrozole, in which the impurity content of relatedsubstances of structures (6) & (7) is less than 0.1%


8. The compound of claim 7, wherein the hydrocarbons added to thealcoholic solution are anti-solvent and are selected from a groupconsisting of aromatic hydrocarbons, and aliphatic hydrocarbons.
 9. Thecompound of claim 8, wherein the hydrocarbons are selected from a groupconsisting of C₁-C₁₀, straight chain, branched and cyclic hydrocarbons.